What is the appropriate antibiotic treatment for a suspected bacterial toenail infection?

Antibiotic Treatment for Toenail Infections

Critical First Step: Confirm Bacterial vs. Fungal Infection

Most toenail infections (>90%) are fungal, not bacterial, and require antifungal—not antibiotic—treatment. 1 You must obtain mycological and bacteriological confirmation before starting any therapy, as treating a fungal infection with antibiotics will fail completely. 1

Diagnostic Confirmation Required:

• Obtain nail specimens for KOH preparation, fungal culture, and bacterial culture with sensitivity testing before initiating treatment 1, 2
• Look for clinical clues: Green or black nail discoloration suggests Pseudomonas aeruginosa (bacterial), while thickened, yellowed nails with subungual debris typically indicate dermatophyte infection (fungal) 3
• Check for abscess formation: If a painful subungual abscess or hematoma is present, surgical drainage is mandatory before antibiotics will be effective 1

If Confirmed Bacterial Infection

First-Line Oral Antibiotics:

For methicillin-sensitive Staphylococcus aureus (the most common bacterial nail pathogen), use dicloxacillin or cephalexin as first-line agents. 1

• Dicloxacillin provides anti-staphylococcal and gram-positive coverage 1
• Cephalexin is an effective alternative to dicloxacillin for Staphylococcus aureus infections 1
• Duration: Continue oral antibiotics for 7-14 days depending on severity 1

Special Bacterial Pathogens:

• Pseudomonas aeruginosa (green/black nail): Treat with ciprofloxacin 3
• Other gram-negative or gram-positive bacteria: Treat according to culture and sensitivity results 3

When Antibiotics Alone Are Insufficient:

• Drainage is mandatory if an abscess is present; continue oral antibiotics post-drainage to prevent recurrence 1
• Partial or total nail avulsion may be required for painful subungual abscess or hematoma before antibiotics will work 1
• Reassess after 2 weeks: If no improvement, obtain repeat bacterial cultures with sensitivity testing 1

If Fungal Infection (Most Likely Scenario)

First-Line Antifungal Treatment:

Terbinafine 250 mg daily for 12 weeks is the first-line treatment for toenail onychomycosis caused by dermatophytes, with superior efficacy and lower relapse rates compared to all alternatives. 4, 2

• Pre-treatment requirements: Obtain baseline liver function tests and complete blood count before starting terbinafine 4
• Contraindications: Active or chronic liver disease, lupus erythematosus 4, 2
• Efficacy: 70% mycological cure rate, 38% complete clinical cure (0% nail involvement) at 48 weeks 2
• Evidence quality: Grade A-I (highest quality) 5

Alternative Antifungal Options:

• Itraconazole pulse therapy: 400 mg daily for 1 week per month, 3 pulses for toenails; requires liver function monitoring for treatment >1 month 4
• Fluconazole: 3-6 mg/kg once weekly for 18-26 weeks (second-line, dose not well-established) 4
• Griseofulvin: Poor cure rates and high relapse rates; only use when newer agents are contraindicated 4

Common Pitfalls to Avoid

• Never treat empirically without confirming bacterial vs. fungal infection—this is the most critical error 1
• Do not use antibiotics for fungal infections—they require entirely different treatment with terbinafine or itraconazole 1
• Do not skip surgical drainage if an abscess is present—antibiotics alone will fail 1
• Monitor for hepatotoxicity with terbinafine: Warn patients to report persistent nausea, anorexia, fatigue, vomiting, right upper abdominal pain, jaundice, dark urine, or pale stools 2
• Warn about taste/smell disturbance with terbinafine: Can be severe, prolonged (>1 year), or permanent; discontinue if symptoms occur 2